1. Field of the Invention
The present invention relates to a material of a negative electrode which can be doped/de-doped with lithium and a non-aqueous-electrolyte secondary battery.
2. Description of the Related Art
Since electronic technology has progressed in recent years, electronic apparatuses having small sizes have been developed, which include camcoders, portable telephones, laptop computers and so forth. As a portable power source for operating the above-mentioned apparatus, development of a secondary battery exhibiting a small size and light weight and having a high energy density has earnestly been requested.
As a secondary battery which is capable of meeting the above-mentioned request, nonaqueous-electrolyte secondary batteries have been expected. The nonaqueous-electrolyte secondary batteries contain light metal, such as lithium, sodium or aluminum, each of which is theoretically capable of generating high voltages and which have high energy densities, the light metal being contained to serve as a material for forming a negative electrode. Among the foregoing nonaqueous-electrolyte secondary batteries, the nonaqueous-electrolyte secondary battery containing lithium, which is employed to form the negative electrode, can easily be handled. Moreover, the foregoing nonaqueous-electrolyte secondary battery is able to realize a large output and a high energy density. Therefore, the nonaqueous-electrolyte secondary battery containing lithium has energetically be researched and developed.
When light metal, such as lithium, is as it is employed as the material for forming the negative electrode of the nonaqueous-electrolyte secondary battery, light metal in the form of dendrite can easily be deposited on the negative electrode. As a result, the density of electric currents is considerably raised at the leading ends of the dendrite. Therefore, there arises problems in that the cycle operation life is shortened because of decomposition of the nonaqueous electrolyte and internal short circuit of the battery takes place because the dendrite is excessively enlarged.
To prevent deposition of metal in the form of the dendrite, graphite materials of a type using intercalation reactions of lithium ions into graphite in place of simple use of the lithium to form the negative electrode have been employed. As an alternative to this, carbonaceous materials of a type using doping/de-doping lithium ions into pores have been employed.
However, the graphite material using the intercalation reaction involves a limit imposed on the capacity of the negative electrode as regulated by composition C.sub.6 Li of the interlayer compounds between first-stage graphite layers. The carbonaceous material using doping/de-doping encounters industrial difficulty in controlling the fine pore structures. Moreover, the specific gravity of the carbonaceous material is undesirably lowered. Therefore, the carbonaceous material cannot effectively enlarge the capacity of the negative electrode per unit volume.
Because of the foregoing reasons, carbonaceous materials obtained at present are considered difficult to be adaptable to an operation of electronic apparatuses for longer time and rise in the energy density of the power source. Therefore, development of a material for the negative electrode having excellent performance for doping/de-doping lithium has been required.